Chronic graft versus host disease (cGVHD), a systemic autoimmune syndrome, remains the major cause of morbidity and mortality of long-term survivors of allogeneic hematopoietic cell transplantation (HCT). The long- term goals of our project are to dissect the cellular and molecular mechanisms of cGVHD pathogenesis, and to develop effective therapies for prevention and treatment of cGVHD. The proposed studies will dissect the mechanisms whereby graft and de novo- generated CD4+ T cells interact with graft and de novo-generated B cells to induce cGVHD and clarify whether blockade of GC formation can prevent induction of cGVHD. We have recently developed a cGVHD model using MHC-mismatched C57BL/6 donors and BALB/c recipients. Results with this model closely reflect the transition from acute to chronic GVHD and characteristic features observed clinically in patients. Donor CD4+ T cells induced cGVHD in the presence or absence of host thymus. But donor CD8+ T cells induced cGVHD only in recipients with a functioning thymus. Donor CD8+ T cells damaged thymic negative selection, resulting in generation of autoreactive CD4+ T cells that mediate cGVHD. Donor B cells from the graft and new B cells generated de novo from the engrafted marrow after HCT augment induction of cGVHD by donor CD4+ T cells. Abnormalities in extrafollicular and follicular CD4+ T and B cell interactions have both been shown to be involved in systemic autoimmune lupus pathogenesis. Our preliminary studies using transplants from donors whose B cells are BCL6-deficient and cannot give rise to follicular germinal centers (GCs) did not reduce cGVHD severity at all. On the other hand, results from other investigators have suggested that GC formation is necessary for induction and maintenance of cGVHD. This project is designed to test two related hypotheses relevant to the role of interactions between CD4+ T cells and B cells in the pathogenesis of cGVHD. 1) Mature donor CD4+ T cells from the graft interact with mature graft B cells and de novo-generated B cells after HCT to induce cGVHD in the absence of GC formation, although certain interventions that disrupt interactions between CD4+ T and B cells can prevent cGVHD. 2) Interactions between de novo-generated CD4+ T and B cells may lead to GC formation, but autoimmunity rapidly destroys the GCs and lymphoid tissues. Thus, interventions that block GC formation are not expected to prevent cGVHD. Again, however, disruption of interactions between CD4+ T and B cells can prevent cGVHD. The proposed studies will provide new insights into how CD4+ T and B cells interact to induce and perpetuate cGVHD and will lead to the development of novel regimens for prevention and treatment of cGVHD.